Polycarbonate resins are excellent in transparency, impact resistance, heat resistance, dimensional stability, etc. and used in various fields as versatile engineering plastics. In particular, polycarbonate resins are often used in the optical field, taking advantage of excellent transparency thereof.
General polycarbonate resins are derived from bisphenol A (hereinafter abbreviated as BPA) and have a relatively high refractive index (nD, 1.59), and therefore are used as optical lenses, in particular, as spectacle lenses mainly in North America because of excellent impact resistance thereof (Non-Patent Document 1).
For example, in the spectacle lens industry in Japan, products having a refractive index (ne) of lower than 1.60 are regarded as middle refractive index lenses having a large thickness, and therefore, it has been desired to increase the refractive index of polycarbonate lens to 1.60 or higher and to enhance the added value thereof as a thin-type high refractive index lens.
Under such circumstances, it was found that a polycarbonate derived from 1,1-bis(4-hydroxyphenyl)-1-phenylethane (hereinafter abbreviated as BPAP) has a refractive index higher than 1.61. Meanwhile, a BPAP-type homopolycarbonate alone has low molding flowability, and for this reason, it is difficult to carry out lens molding when directly using it. For example, techniques of copolymerizing it with a bisphenol that improves molding flowability to achieve a balance between high refractive index and molding flowability are known (Patent Document 1, Patent Document 2).
However, since an expensive bisphenol is used for copolymerization in these techniques, the manufacturing cost is high, and for this reason, these techniques are not necessarily acceptable in the lens market where price reduction has been promoted. Moreover, a polycarbonate-based lens material having impact resistance in addition to high refractive index and molding flowability has been desired.